I assume you have an oil line going to the gauge if you could blow air into the gauge? I'm don't recall the the set up on a 77 FLH.Do you also have an oil light?
By what you're saying I would guess your gauge is bad.On newer FLH's the sending unit has 2 wires on it,1 for light,1 for gauge.There is no oil line to the gauge.Was this a gauge add on in the tappet screen hole?
tourbox

You need to get a replacement IF the oil is under any pressure and com'n out, it shoulld register.. What is the poundage on the gauge face.... Low number to High number?

You will want to keep it the same... Modern gauges won't do your bike a lot of good. My thinking is that old of a bike gauge has a top number of 15 or 30 pounds... Those early bikes used a LOW oil pressure, so the gauge Has to be the same to register it correctly... Putting in a 0/100 lb gauge and that would probably not register oil at an idle either.

IF it is running out the line and holding the oil line at a horizontal level and the oil pumps-out at least a 1/2 inch from the tube end at an idle,,,, You have oil pressure...